Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks which are admixed with various additive packages and solvents depending upon the intended use of the lubricant. Conventional base stocks include, for example, mineral oils, highly refined mineral oils, poly alpha olefins (PAO), polyalkylene glycols (PAG), phosphate esters, silicone oils, and polyol esters.
Lubricants are oftentimes used under extreme thermal and oxidative conditions, such as in aircraft turbines and in internal combustion engines. Polyol esters have been commonly used as lubricating base stocks in automobile and aircraft engine oils. The inherent thermal and oxidative stability of polyol esters is relatively high compared to other base stocks (e.g., mineral oils, poly alpha olefins, and the like). However, even these synthetic esters lubricants are subject to oxidative degradation and cannot be used for long periods of time under oxidizing conditions without further modification. It is known that this degradation is related to the oxidation and hydrolysis of the ester base stock. Such deterioration is manifested by the formation of products of oxidation such as sludge and varnish like deposits on the metal surface, and by viscosity and acidity growth.
Conventional synthetic polyol ester engine and turbine oil formulations require the addition of anti-oxidants (also known as oxidation inhibitors). Such anti-oxidants include arylamines (e.g. dioctyl phenylamine and phenylalpha-naphthylamine), and the like. Anti-oxidants reduce the tendency of the ester base stock to deteriorate. Upon thermal oxidative stress, a weak carbon-hydrogen bond is cleaved resulting in a unstable carbon radical of the ester. The role of conventional antioxidants is to transfer a hydrogen atom to the unstable ester radical and effect a "healing" of the radical. The following equation demonstrates the effect of the antioxidant (AH): EQU AH+ROO.cndot..fwdarw.A.+ROOH
The anti-oxidant molecule is converted into a radical, but this radical (A.) is far more stable that that of the ester based system. Thus, the effective lifetime of the ester is extended. When the added anti-oxidant is consumed, the ester radicals are not healed and oxidative degradation of the polyol ester composition occurs.
Frequently replacing the lubricating oil or adding an antioxidant thereto to suppress oxidation increases the cost of maintaining internal combustion engines and aircraft turbines. It would be desirable to have an ester base stock which exhibits substantially enhanced thermal and oxidative stability compared to conventional synthetic ester base stocks, and wherein the ester base stock does not require frequent replacement due to decomposition (i.e., oxidative degradation). It would be economically desirable to eliminate the amount of antioxidant which is normally added to such lubricant base stocks.
U.S. Pat. No. 5,665,686, which is incorporated herein by reference, describes high hydroxyl content polyol esters that are useful as lubricants since the highly branched polyol ester backbone permits the high hydroxyl ester to act in a manner similar to an anti-oxidant. The high hydroxyl content causes the thermal and oxidative stability of a polyol ester to drastically increase as measured by high pressure differential scanning calorimetry (HPDSC) by providing an intermolecular mechanism which is capable of scavenging alkoxyl and alkyl peroxides, thereby reducing the rate at which oxidation degradation occurs as compared to similar, low hydroxyl content polyol esters.
In addition, U.S. Pat. No. 5,665,686 also discloses that high hydroxyl content polyol esters exhibit lower friction coefficients and wear volume than fully esterified polyol esters.
The present invention provides a method for producing glycerol di-esters which can be used as a base stock lubricant. The present method is unique since it can quantitatively produce an essentially pure di-ester with virtually no mono-esters or trimesters. Previously, such di-esters were produced in combination with mono-esters and tri-esters by, for example, the acid esterification of glycerol as disclosed in EP 0 739 970 A1.
The thermal and oxidative stability of the di-ester compound of the present invention eliminates or reduces the level of anti-oxidant which must be added to a lubricant, thereby providing a substantial cost savings.
Also, since the method of the present invention produces essentially pure di-esters, the reproducibility and consistency of the lubricant is greatly enhanced compared to mixtures of polyol esters where the esterification process results in varying concentrations of mono-esters, di-esters and trimesters. The important anti-oxidant hydroxyl content of the polyol ester mixtures produced by the acid-alcohol esterification process will be much more varied than the essentially pure di-ester.
Furthermore, the reaction conditions of the present invention are mild compared to the alcohol-acid esterification process. The alcohol-acid esterification process requires a reaction temperature of over 100.degree. C., and typically over 200.degree. C., wherein the present reaction method only requires temperatures of about 70.degree. to 90.degree. C.
Moreover, the di-ester product is readily recoverable from the reaction mixture. Reactants not consumed in the process can be treated by a water washing, vacuum drying and a mild wiped film evaporator treatment. On the other hand, the alcohol-acid esterification isolation process is much more complicated and expensive. For example, if a catalyst is used in the alcohol-acid esterification, it is typically removed by filtering the reaction mixture and distilling the resulting filtrate. If it is difficult to remove the catalyst, a filter aid may be needed. Also, the unreacted alcohol (glycerol) may adversely affect the performance of the lubricant by deteriorating the anti-wear properties. Therefore, it is preferable to reduce the alcohol content of the reaction mixture by distillation which is expensive. Furthermore, it is unlikely that the unreacted alcohol can be removed completely by distillation. Thus, the method according to the present invention is much simpler, results in essentially a pure di-ester with no alcohol content, and is economically more efficient than the conventional alcohol-acid esterification process.